Dyeing of keratin-containing fibers, especially human hair is a marriage between art and science. It is a clear example of tuning an aesthetically desired result with the ability to deliver that result by way of a chemical reaction that takes place at the time of use; there is no going back to the laboratory and trying again if the outcome is not what the user desires. As such, the formulation of a keratin-containing fiber coloring composition must not be susceptible to the slight to moderate variations in product use instructions that may occur during application.
In addition, hair coloring is not done in a void; the compositions are applied to keratin-containing fibers that vary greatly from subject to subject due to an array of variables, inter alia, condition of the hair and scalp, previously applied treatments, and the morphology of the fibers themselves. Hair can be fine, thick, dense, or sparse. Therefore, any composition for use in dyeing human hair must be able to evenly coat the fibers, as well as to permeate the surface of the fibers themselves regardless of the condition of the fiber itself.